The objectives of this project are to determine health effects of the volcanic eruptions of Mount St. Helens under controlled conditions and to generate a comprehensive scientific data base. This information is expected to aid state and federal authorities substantially in assessing the health hazards of chronic exposure to volcanic ash. In the inhalation experiment rats will be exposed to volcanic ash (50 to 5 micrograms/l) and quartz (50 micrograms/l; positive controls) for up to 24 months to investigate the fibrogenic (pneumoconiotic) properties of the ash as a function of aerosol concentration and exposure time, compared with sham-exposed controls. The pulmonary deposition and clearance study will involve a single nose-only exposure of rats to neutron-activated ash to provide information on the fate of the ash deposited in the alveoli, quantity of initial pulmonary ash deposition under defined exposure conditions, biological half-life of the pulmonary ash burden, pulmonary clearance rate, and time of complete clearance. In vitro tests with macrophages will determine the cytotoxicity of the ash relative to other materials including standards. Thorough physicochemical characterization of the ash will permit correlation of biological effects with its properties, such as particle size, mineralogical composition and trace element abundances.